The planet or possibly a un-ignited proto-star Nibiru is the homeworld of or possibly a colony of the Anunnaki race as well with them also theorized to come from the Pleiades considering they were reported to look like Humans but only taller than normal humans. The Hebrew religion never names this race it simply calls them "Giants", but tries to distinguish them from the Nephilim which were other giants. There's also the belief that possibly the Nephilim in some version of the Hebrew scripture that the Nephilim are the half-breed race of children of the Anunnaki and not a separate account of a similar race entirely. This is based in ancient Mesopotamian and Hebrew religions. Although these accounts date back to the 2000s BC and the 1000s BC, according to all of these scriptures the Aliens come to earth around 4,000 BC and start a war known as "the War of the Gods".
According to these Mesopotamian legends, Nibiru is originally from a solar system near the star Sirius. Somehow, the planet was ejected from that solar system and caught by ours. As it's new orbit placed it very far from the sun, the Anunnaki needed a plan to retain heat for the planet. They decided to do this by lining the upper atmosphere with gold dust. To do this, they needed a large supply of gold. An expedition lead to Earth found that it was of high gold content, so they began mining for it.
Crash with TiamatEdit
At the time before Nibiru entered our solar system, the legends maintain that our planet Earth (called Tiamat to the ancient Babylonians) was almost twice as large and had a different orbit than today. The planet Nibiru came in on its highly unusual orbit, and a moon orbiting it impacted in the pacific ocean, apparently destroying an entire continent. Although we have never found evidence of that continent's existence, some believe this to be how the Moon was created, because scientists are certain the Moon was formed by the Earth. The resulting displacement of crust around that area of impact (and some in other parts of the planet, no doubt) made earth much smaller than it is today. The knowledge of this event apparently came to the ancient peoples thousands of years after the fact due to the spirits of their dead Anunnakian gods that died on earth during the War of the Gods. although in some versions of the Annunaki legends they revisited the earth after the War in Heaven and then help build up Mesopotamian civilization directly instead of just the spirits of Anunnaki helping the ancient Mesopotamians.
Rob Solarion's Lecture in Texas